01 - Place the cubed potatoes in a large pot and cover with cold, generously salted water. Bring to a rolling boil over high heat, then reduce to a gentle simmer. Cook until the potatoes are fork-tender, approximately 12 to 15 minutes. Drain thoroughly and set aside.
02 - While the potatoes cook, melt 1 tablespoon of butter in a medium skillet over medium heat. Add the red bell pepper, jalapeño, green onions, and garlic. Sauté until the vegetables are softened and fragrant, about 4 to 5 minutes. Remove from heat and reserve.
03 - Return the drained potatoes to the pot. Add the remaining butter and pour in the warm milk. Using a potato masher or hand mixer, mash until smooth and creamy, working to your desired consistency.
04 - Fold in the sautéed vegetable mixture, shredded cheddar cheese, sour cream, ground cumin, smoked paprika, chili powder, salt, and black pepper. Stir until all ingredients are evenly distributed and the cheese has fully melted into the potatoes.
05 - Taste the finished mash and adjust salt, pepper, or chili powder to your preference. For additional heat, consider adding a pinch of cayenne pepper or retaining some jalapeño seeds in the sauté.
06 - Transfer the mashed potatoes to a warm serving bowl. Garnish generously with chopped fresh cilantro and additional sliced green onions if desired. Serve immediately while hot.